How does EOS must be launched by Epic Games Launcher?

Hello, I hope you’re doing well. I have a question about Unreal Engine EOS. In the documentation (see screenshot), it says that if my game uses the EOS login method “accountportal,” it must be launched from the Epic Games Launcher (see highlighted section in the screenshot). What does that mean? Can’t I use an Unreal Engine source build? Thank you for any help!